Acceptor binding energy in GaN and related alloys

The recent achievement of p-type doping in GaN epitaxial films emphasizes the importance of the acceptor ionization energy with relevance to the application of group III nitrides in visible-light-emitting devices. Measured values of approximately 200 meV suggest that doping efficiencies at room temperature may be no more than 1%, resulting in undesirable series resistance in LEDs and lasers. Consideration of the effective mass hydrogen model of the acceptor ground state in GaAs, GaP and GaN suggests that this large value of GaN results from the strong electronegativity of the nitrogen atom and the corresponding reduction in dielectric constant, compared with the other compounds. This is further emphasized by the need to use the high-frequency dielectric constant rather than the static value when the ionization energy is greater than the TO phonon energy. Using in ( infinity )=5.35 and an effective mass of 0.4 m provides good agreement with the measured ionization energy of GaN, this value of m n being consistent with estimates based on recent hole mobility measurements. It is also consistent with smoothly increasing hole mass in the sequence GaAs (0.33 m), GaP (0.37 m), GaN (0.40 m). Finally, the opportunity is taken to speculate about likely values of acceptor ionization energies in AlGaN and AlGaAsN alloys.